1. Field of the Invention
The present invention has application as a heat transfer system specifically adapted for use in the field of heating and air conditioning.
As the world's sources of fossil fuels such as oil and natural gas diminish, increased emphasis will be placed upon the use of electricity, which may be generated through the burning of coal or by nuclear reaction, in order to provide space heating of homes, commercial structures, and factories. One of the most energy-efficient systems utilizing electricity for the aforesaid heating applications is the wellknown vapor compression type heat pump. Although heat pumps of this type have been known for many years, and are today experiencing considerable popularity, such heat pumps have always experienced a common operating characteristic which is undesirable. That is, as the ambient heat source of the heat pump falls in temperature, the capacity of the heat pump system decreases drastically; while, at the same time, the heating needs of the space being conditioned increase. This problem has led to the extensive use of supplemental electrical resistance type heating, which is much less efficient than heat pump operation, and, in northern climates, has made heat pump usage less desirable.
2. Description of the Prior Art
The problems associated with low ambient heat pump operation have not gone unnoticed throughout the years, and several solutions to the problem have been proposed.
A first solution has been the provision of multiple compressors in the heat pump circuit which are selectively operable in series or compound relationship during low temperature operation; but which may be operated either in parallel, or one of the compressors may be rendered inoperative, during high temperature operation. Systems of this general character are disclosed in U.S. Pat. Nos. 2,919,558; 2,869,335; 2,938,361; 3,077,088; and are also described in articles appearing in Power Engineering, June, 1958, pages 72-74; and in Refrigerating Engineering, May 1956, pages 48-51.
A second solution to the aforementioned problem is disclosed in U.S. Pat. No. 3,392,541. The system disclosed in FIG. 1 thereof includes two separate refrigerant circuits which are operable in a first mode wherein evaporators 22 and 22a provide cooling, and in a second cascade mode wherein one of the circuits provides defrost or heating while the other continues to provide cooling. No provision is made for operating in a single stage heating mode. The system disclosed in FIG. 5 of this patent is similar to that of FIG. 1 except the second refrigerant circuit thereof is also reversible such that the system is always operable in a cascade mode to provide either cooling or defrost, or heating. Once again, no provision is made for single stage heating.
Yet a further solution is proposed in U.S. Pat. No. 2,707,869 wherein a system is disclosed having two refrigerant circuits, one of which is operable to absorb heat from a heat storage reservoir and transfer same to a conditioned space; the second of which is operable to absorb heat from an ambient heat sink and transfer same to the aforesaid heat storage material. At low ambient temperatures, the second circuit is rendered inoperable unless the temperature of the heat storage material reaches an unacceptably low level. Further, no provision is made for operating the first refrigerant circuit in a mode such that it may absorb heat directly from the ambient heat sink at such time as its temperature is at a relatively high level.